It has long been known that strokes and other circulatory disorders can be the result of embolic particles carried by the bloodstream. More recently, it has become evident that large numbers of emboli occur during surgical procedures and that higher loss of neurological and physiological function is associated with higher numbers of emboli in the bloodstream supplying the brain. During surgery, clots may form in the blood, air may enter into the bloodstream, or tissue fragments may break loose or become dislodged. These emboli are carried by the blood into increasingly smaller arteries until they become lodged and obstruct the flow of blood. The mount of damage that results depends on the size of the emboli, the point in which it lodges in the blood flow, the amount of blood leaking around the emboli, and how blood is supplied by collateral paths around the obstruction. The resulting functional deficit depends in part on the composition of the emboli. For example, air may be reabsorbed in a short time, clots may dissolve (particularly if blood-thinning drugs are present), while particles composed of plaque and body tissue may not dissolve at all. Therefore, it is important to have non-invasive instrumentation that can determine the composition of emboli and estimate their size as well as count them so that appropriate medical management decisions can be made.
The present state of the art for non-invasive detection of emboli relies primarily on Doppler ultrasound to monitor blood vessels for the presence of emboli. When an emboli passes through an ultrasound beam, the change in acoustic reflectivity causes a strong reflection which can be detected by the ultrasound receiver. The number of embolic events can be counted by monitoring the amplitude of the Doppler signals. Embolic events are deemed to occur whenever the amplitude of the Doppler signal exceeds a predetermined threshold.
While the amplitude of the Doppler signal has been used effectively to detect and count emboli in the bloodstream, it does not provide sufficient information to characterize emboli based on composition and size. The amplitude of the Doppler signal may be affected by the size, composition, and/or shape of the emboli. The amplitude is also a function of transducer beam shape and of the location of the embolus relative to the sound beam. Because of these variables, the amplitude of the Doppler signal alone does not provide unambiguous information about the composition or size of the emboli. For example, the amplitude of the Doppler signal produced by a small ,gas bubble may be stronger than the signal from a large clot. In certain surgical procedures, the composition of the emboli (e.g., solid or gaseous) may be an important factor in determining the clinical procedures necessary to avoid complications arising from the presence of the emboli.
In the past, characterization of the emboli based on the time waveform of the Doppler signal has been attempted. However, the time waveform is highly variable with blood velocity, transducer beam shape, the position of the embolus within the ultrasound beam, and the composition of the embolus. For example, the amplitude of the time waveform may be effected by the position of the emboli within the sound beam (with emboli near the center of the beam producing larger amplitude signals than emboli near the edges of the beam) as well as the size and composition of the emboli. Similarly, the duration of the time waveform may be effected by the blood velocity, as well as the position of the emboli within the sound beam (with emboli near the ultrasound beam focus producing a shorter duration signal than emboli away from the focus). The interdependence of these variables makes it difficult to extract reliable information from the time waveform concerning the size and composition of the emboli.
Recently, attempts have been made to characterize emboli by using two or more ultrasound beams with varying frequencies to differentiate between gaseous and solid emboli. Such a system is described in U.S. Pat. No. 5,248,015 to Moehring, et al., and in an article entitled, "Pulse Doppler Ultrasound Detection, Characterization and Size Estimation of Emboli in Flowing Blood" published in Transactions on Biomedical Engineering, Vol. 41, No. 1, January, 1994. This method uses the ratio of the acoustic power backscattered from the embolus to that of the moving blood surrounding the embolus. This ratio is called the "embolus-to-blood ratio" (EBR). It is postulated that the ratio of EBR at two different frequencies will be one value for solid emboli within certain operational limits, and that the ratio of EBR at the same two different frequencies for gas emboli will be another value. Thus, emboli can be characterized by comparing the ratio of the EBR of the reflected signals at two or more different frequencies.
While the characterization of emboli based on the "embolus-to-blood ratio" of multiple signals is promising it has several drawbacks. First, this method requires measurements at two or more frequencies which increases the complexity of the system. Secondly, this method may have difficulty with emboli that resonate. The resonance of the emboli will vary with size and may alter the amplitude ratio at different Doppler frequencies.
It has long been known that the reflection coefficient of particles will vary based on the composition of the particles. This natural phenomenon is true for emboli as well as other types of materials. For emboli that are less dense that blood (i.e., gas and fat particles), the reflection coefficient is negative. The reflection coefficient will be positive for emboli which are more dense than blood (such as clots and plaque). A negative reflection coefficient means that the phase of the reflected signal will be inverted 180 degrees from that of the incident signal. If the phase of the incident signal is known, then it is possible to compare it with the phase of the reflected signal to determine if the unknown embolus is of higher or lower impedance than blood.
With a pulse Doppler or continuous wave Doppler signal, it is difficult to determine the absolute phase of the reflected signal because of the nature of Doppler detection. The Doppler detection system looks at the change of phase with time to measure the frequency shift but does not utilize absolute phase. In order to determine absolute phase, the phase of the incident wave must be known very precisely at the range of the embolus. Thus, the range of the embolus from the transducer must be known within a small fraction of an acoustic wavelength.
It is not possible for continuous wave Doppler to measure range. Pulse Doppler can measure range but not with sufficient accuracy since pulse Doppler systems are typically narrow bandwidth, often in the 20 percent to 30 percent range. This means that the received echo signal will build up gradually over several cycles of the carrier frequency so it is very difficult to determine precisely when it starts. While the signal is relatively large after several cycles, it is necessary to project this phase back to the precise beginning of the pulse with an accuracy of better than one-quarter of a cycle. Since current Doppler systems do not have this range resolution accuracy, they have not been able to use the phase of the reflected signal and have had to revert to less direct methods to predict the impedance of the emboli.